Waveform averaging rc circuit

ABSTRACT

A waveform averaging circuit composed of resistance, capacitance and diodes yields an output which is similar to an inductorcapacitor filter in rectified outputs of a transistor switching regulator. The LC filter is employed in a load output while the RC filter is employed in a feedback sense circuit for control of the regulator. The RC circuit provides a cost and weight reduction and provides less phase shift than would result from the employment of an LC filter in its stead.

States Patent 1191 1111 3,859,590 Cielo et al. Jan. 7, 1975 [54] WAVEFORM AVERAGING RC CIRCUIT 3,697,852 10/1972 Gerbitz 323/DIG. l 3,733,519 5/1973 Griffe 323/DlG. l X [75] lnventorsl Robe" Clem; 3,772,588 11/1973 Kelly Z1211 321/2 x Moorman; Suryanarayana Krishnappa Rao, all of Kingston, N.Y. Primary ExaminerGerald Goldberg [73] Assignee: International Business Machines Corporation, Armonk, N.Y. 221 Filed: July 30, 1973 [57] ABSTRACT [21] Appl. No.: 383,741 A waveform averaging circuit com osed of resistance,

P capacitance and diodes yields an output which is similar to an inductor-capacitor filter in rectified outputs [52] Cl ggf gq iggyg of a transistor switching regulator. The LC filter is em- [51] Int Cl Gosf ployed in a load output while the RC filter is em- [58] Fieid l 22 ployed in a feedback sense circuit for control of the regulator. The RC circuit provides a cost and weight 321/2 307/265 333/70 CR reduction and provides less phase shift than would re- [56] References Cited suit from the employment of an LC filter in its stead. UNITED STATES PATENTS 10 Claims, 2 Drawing Figures 3,602,80l 8/1971 Williamson 323/DIG. l

- and more.particularly tofan irnproved filter circuit ffor 17 I providing an'joutputwhich corresponds to'thehalf 2; Descriptionofthe- Prior Art,

' Filters are widely used in'power supplies-andelse 1Y0 aforesaid potential source comprises a capacitor which duction'of AC componentstherein ln'th base of a' DC.

' power supply it is "often desired "that the AC ripple 0th-' erwise'pltescnt in th' output be removedand in the case f'I/tof-ai transistor switching regulator, the output of the 15 tion, a load i'sprovided across the capacitor of the RC pulses, andto translate thi svafr'i'ation 'into'control of the u 'a feedback sense signal for the control; of .a transistorvided circuits which sensethefloadoutputof theregu H v p g later so as to obtain a signal indicativeof needed. ;Still another object of theinvention is to provide a changes in'the dutyicycle of the switchingntransistor.

' DC decoupling device,Lprovide DCiiisolation,between" -the'input and output of theregulato'r'.

signal. Thus, the signalhas DC i'solationlfrom; the: lea

' derives the sense signal.froni-"theiadditional sec'ondar track-the load output ina suff ciently parallel -fashion'-:. I,

' -to provide a meaningfulc'ontrol signal. Accordingly; a I filter'circuit has been utilized injthe'se is similar to the .filterl-circuit'lin {the'loadoutput circa" for circuitsgso as tofhave a similar electrical ha i a' tier-is tic toyield the desired tracking-functionlfihis:;dictates;.

have the advantage that theresistorjeiriployed is-usually 3 jlig'hter and less expensivethanith'einductor of arr-LC in employnientis wherein. the current drain i sfverjylight'jf.

, 'Asthe, load isincre ased the potentialjacrjoss;theyfilter '1 1 capacitorfgradually decreases, andiat'in'o 'pointi the f output ofthefilterinherently/ an average'ofith nput'.;'

tial of a first predetermined level'so as to apply that'p -g'tentia'l toth RC filter, and means adapted to apply a -'s ecorjidj potential level 'to the RCfilter during applica- ;-.tion of inputpotentialo f a second predetermined level. 7 "According-to another aspect of the invention, the electrical'function circuits 5 means to apply th'effsecond'potential level to the input -'of the filtencomprises a potentialsource applied to the input'ofthe filter in the absence of an input of said first potential level.. i I .r 'According to still another aspect of the invention, the

V w vEFoiiMfAvaRAmNc Re'cmcuiir} I BACKGROUND OF THE: NYE ION f l. Field of thelnvention This invention relates to wave average'of the' 'inp u t applied thereto .where to smooth an elec'tricallsignal by remqval oii r'e is charged to the second level of input during a portion of the input cycle subsequent to that portion during which'thefirst potential level is applied to the filter.

In-accordance with still another aspect of the inventransistor switch may provide energyjonlyj in'pulsating form and it becomes the function of the" output filter to convert" this pulsating energy into the desired. direct the filterforachieving a'scaled average output across current-level. ln atransist or'switchingregulator, theenthe capacitor. 3 l e l u 1 tire control is frequentlypredicated'kin-varying the du- 0 'ln 'accordancewithanother aspect of the invention, ration, as distinct from the amplitude; of the energy -a sense circuit is provided in a'tran'sistor switching regulator in a'ccordancewithone or m'ofe'o'f the'foregoing jQaspectsofthe invention.

' Ac'cordingly, an object of-the invention is to provide filter--which,;together with the filter resistor of the RC filter, forms a voltage divider loading the capacitor of output DCle'vel, itis necessary'tha't the output filter 3 of 1 an integrating or averaging'f-kihd' as opposed to, a

peakresp'onsiveQfilter. Accordingly, LC filters are an improved signal averaging network. a

S'till'another object of the invention is to provide an widely used in thiscircuit employment-Q For providing 7 I v "RC network which will provide'a voltage output which corresponds to'lthe average of the voltage input ofa ".lgivenpolarity'applied thereto.v

switching regulato rofthisikind," the prior art has pro-.

u g 7 sense circuit ina transistojrlswitching regulatorwhich This arrangementdoes notjabsent the additionoflsomel j i s characterized by an-irnproved networkas aforesaid. Still-anotherobject ofthefinvention isto. provide a u I 'transistor, switching regulatorlwith a isens e circuit as To provide'DC isolation between the, input and' out; aforesaidiwhich;provides'a sensesignal which correput of such a regulaton-thei transistor switching :regula vsponds to the average outputfof the transistor switching tor has been coupled tolthe output load circuitsyby-ani regulator and which hasa'n improved phase relationisolating transformer, and additional windingson the' s'hiptojthe input of the transistor-switching regulator. transformer -has'beent'util i zed to provide the"fe'edback J output ci'rcuit, butit isnece'ssary that the-c'ircuitwh followingdescription"of a preferred embodiment of the I invention as illustrated intheac'c'ompanyingdrawings BRIEF-DESCRIP-j" rjH .oaAwiNos' Prof 1' is" a .sc'henia't zdi u tw i the-invention can" be used tojadvanta'ge.

thenfemployment of {LC filterin the; se nsejcirc'ui a which is u'ndesirably large, heavy andfexpen'siv' f Resistor-capacitor filters have beenl' used widelyan'd.

'duringioperation thereof g j DETAILED-DESCRIPTION filter. Since the resistor. of an R C "filter is dissipativeyacross'input terminals 10, 12. ln-the'circuit-shown, volt- Ithis' kind of filter is usually seen, 'as';'a' peak filtering, filter age'E is xapositive direct current voltage'which may be }unr egul'at ed.or only poorly regulated, and afunction of .i a ge at t'erminals- 14,16 forc'onn ection to 'a useful'load.

-S UM MARYD F THa-niveisiflojN c mer-2 ransfor mer 20 has an output secondarywinding 26 poled to conduct during application ofiajn input pote,n,-,.-

l The foregoing andotherobjects, features. 'and 'ad'vantage's'of'the'presentinventionwillbe apparent from the H irepre sent'ation'ojfa transistor swit'ching -regulator in which an improjved network of i FIGQZ isa waveform diag'ra'mdndic ative of voltage relatjiforiships' atvarious pointsin the' circuit of FIG. 1-

- In the circuitiof Pro. 1, supply lvoltage E is applied" the circuitisito provide .a better regulatedoutput volt- Thelin p ut voltageE isapplied acrossprimary winding t -l8. ,of a-transformer 20'by'a switching transisto'r'zzi... A e a, l g fwhichisope'rated in an on-.offmanner byp'ul'sewidt'h According to one aspect of the present int/enema- I network isprovided which yields an output whic h;i s the r u I half-waveaverage of the input applied"theret0,theneb- 615 w,hi'ch is connected through rectifier diode to' an work including an RC filter'h'a'ving antiinputelement filtercomprising inductor'30,zcapacit0r 32 and freewheeling diode 34. Whe.n switch 22 is turned onto or" near-saturation, voltage E is applie dacross' primary 18,] v j and a voltage is induced in secondary 26 which is coupled through diode 28 to inductor30. The resulting current flowing in inductor 30 supplies current to any load connected across output terminals 14, 16 and also adds to the charge in capacitor 32 and the storage of energy in the field of inductor 30. When switch 22 is turned off, the current through primary 18 is interrupted and a reverse voltage is induced in secondary 26. A clamp circuit comprising a capacitor 36 in parallel with a resistor 38 may be connected by a diode 39 across primary 18 to absorb the magnetization energy of transformer 20 during the collapse of the magnetic field linking primary 18 when switch 22 is off. During the off time, the reverse voltage induced in secondary 26 is blocked by diode 28 and current in inductor 30 begins to decay. A path for current through inductor 30 is provided by free-wheeling diode 34 which is forward biased at this time.

A function of pulse width control circuit 24 is to vary the on time of switch 22 in a cyclic manner so as to maintain the voltage across output terminals 14, 16 at a predetermined level despite changes in the input voltage across terminals 10, 12. To perform this function, pulse width control 24 utilizes a sense input which is compared in circuit 24 with a reference to control the on-drive time of the base drive signal provided on line 41 to the switching transistor 22. In order to simplify the diagram, the details of pulse width control 24 are not shown. commonly, a circuit of this kind comprises a reference such as a zener diode powered from any convenient source, a differential amplifier to compare the signal fed back on line 40, or a predetermined portion of that signal, with the zener reference, and an on-off control responsive to that difference. The on-off control may, in simple instances, be a simple limit cycle type of operation in which whenever the output of the differential amplifier is of one sign the switching transistor 22 is turned on and when it is of the opposite sign it is turned off, or it may be a more elaborate circuit such as one including an oscillator driven circuit which provides a fixed frequency output which has an on-time responsive to the sense-reference difference. The prior art includes a large number of such circuits in transistor switching regulators, and the details thereof form no part of the present invention.

Transformer 20 provides the advantage that one or more load circuits, such as load circuit 26, 14, 16, can be provided which are direct current isolated from the input circuit 10, 18,22, 12. Thus the load across output terminals such as 14, 16 can have a ground which is independent of that of the input terminals l0, 12. Accordingly, it is desired that the feedback sense signal on line 40, while tracking the voltage applied to the load across terminals 14, 16, be isolated therefror'n so far as ground reference is concerned. To provide this isolation, an independent secondary winding 42 on transformer 20 is employed as the feedback source. This winding 42 is provided with a network in accordance with the invention which yields an output at point D connected to sense line 40 which closely tracks, i .e., is substantially a predetermined linear function of, the output voltage at terminal 14 of the load circuit of the regulator.

This network comprises a rectifier 44 and an RC filter network including input resistor 46 in series and filter capacitor 48 across the output. Another resistor 50 in parallel with filter capacitor 48 provides a relatively at the output of winding 42, less the small forward bias voltage drop in diode 44. 0

Means are provided to bring point C to a second level, such as a logic ground level, when switch 22 is off and voltage of the opposite polarity isinduced in secondary 42, back biasing diode 44. In accordance with a preferred embodiment of the invention, this means comprises a capacitor 52 which'is connected across winding 42 by diode '54 which is poled to conduct during the reverse polarity output of winding 42. The reverse charge thus provided at terminal B of capacitor 52 is connected to one end of a resistor 56. The opposite end of resistor 56 is connected to point cat the input of filter resistor 46, which is clamped to the logic ground bus 58 of the circuit by diode 60. By operation of the connection through resistor 56 and the clamping action of diode 60 point C is brought to a level which approximates that of logic ground bus 58 (more precisely below that level by the amount of the forward bias drop of diode 60) when transistor switch 22 is in its off condition.

FIG. 2 illustrates the aforedescribed operation. When transistor switch 22 is turned on by a forward bias base drive signal on line 41, current flows from terminal 10, through primary 18 and switch 22 to terminal 12, inducing a voltage in the secondary windings, including winding 42. Curve A shows the resulting voltage at point A in FIG. 1, with respect to logic ground bus 58. This voltage +Vl is chiefly a function of the voltage +E applied at terminal 10 of the regulator input; multiplied by the turns ratio of primary winding 18 to secondary winding 42. As shown at 72 in curve C the voltage +V 1, minus the small forward bias drop in diode 44of FIG. 1, is communicated directly to point C at the input of RC filter 46, 48.

As will be explained with reference to the second half of the cycle under discussion, capacitor 52 had been charged in a previous cycle to voltage V2 but the potential at point B rises slowly, as shown at 74, toward the value of point C in accordance with the time constant of the resistor 56, capacitor 52 combination. This constitutes a load on point C but has little effect on the potential thereof during this portion of the cycle, and the RC filter 46, 48 operates to provide a filtered output of approximately +V3 at point D of the filter. The output at point D during this portion of the cycle rises slightly, as indicated "at76 in curve D, but since the time constant of RC filter 46, 48 is relatively large, the slope of this rise is slight,

When pulse width control 24 operates via line 41 to turn transistor 22 off, the potential at point A drops abruptly as indicated at 78 and continues negatively'to a value V2 during the magnetization reset action of transformer 20. The reverse voltage appearing across primary winding 18 at this time is limited by clamp circuit 36, 38 to a value determined by the charge on clamp capacitor 36 plus the forward drop in diode 39. This potential is reflected into secondary winding 42 whereby point A thereof falls to a level of V2. After i 52. Accordingly, filter capacitor discharges slightly I where V, the positive peak voltage'(70)-applied jto pq ir it,

V2 at point A forward b'i'asesdiode 54 and recharges capacitor 52 toapproximately V2,'as indicated at 82.

This voltage at point B is applied to the end of resistor. I v

56 connected thereto so as to pull point C toward value:

sired tracking of sense filter output point D with the usefulload outputs such as at terminal 14 is achieved. Representative values of a typical sense circuit in accordance with the present invention are as follows:

Resistor 56 130 kilo-ohms Resistor 46 17.4 kilo-ohms Capacitor 48 0.68 micro-farads Resistor 50 3.01 kilo-ohms Capacitor 52 1,200 pico-farads V2, until diode 60' .becomes rjforward.- biased and +Vl +40 volts clamps p'oint'C to the levelof logicgro'und busl58, as V2 40 volts indicated at 84 in curve'C. i I, I s a +V3 =+2.5 volts Accordingly, current flows outof filter'c apacitor 48,- T, to microseconds not only through load resistorl 50 .but. backwards T 50 microseconds through filter resistor 46 and resistor 56 into-capacitor -l5 I While the invention has been particularly shown and as indicated at 86, curve D. However, as represented in 0 curve D, the value at point D and thereforethe feedback sense signal on line remains relatively constant at-a value of +V3 throughout the cycle. Preferably the logic power for the pulse width control circuit 24, including its zeneror other reference circuit andithefoutput drive power on line 41 to the base of (switching transistor 22 isderived from a source other th'arifeedback sense line 40. For example','thejt-ransistor -switchingregulatormay have a numberof. outputs similar-to output circuit 14,116, 26,28,310, 32,34, each operated by its own seeondaryfwindingiSuch,otherv circuits are indicatedat 88and one-ofthem-inaybe uti-' lized for providing power to pulsefwidth control 24 as indicated in FIG. l 'at +V'4:'I?hus the'impedance' ofthe sense input of pulse width control 24,-'to-which line 40 ,is connected can'beof relatively. high'orat'leastrelatively steady, value ln such'case'the output of the illustrated sense filter' at point Dis givenby-the ,er'rpression:

R ="thev alue of'load resistor (plust'heinlp utirm' pedance at the connection-of sense line 40tdpulsei width control 24). x

R the value of filter resistor 46. 5

T, the on time of switching transistor 2 2"an'd 1 the overallswitching period'of the regulator. t

- V, the average value, +V3, at point D.

As aforest a ted, the resistor combination 46, 50 con- I stitutes a voltage divider. Thus i-V3 is a predetermined; 5 Y

fraction of the half-,wave'ave'rage of the signal applied: 1

at point A. This provides a step-down which reduces 1" error introducedby-the.forward voltage 'dropsin. diodes 44 and 60, andprovid'esa means to accommodatdthe potential range desired for the feedback'on line 40, to the'higher potential which may-be' available"at point A in a given circuit employmentQlf load resistor 5.0 were eliminated and the current drawn throughsense line 40 ."were made very small, the voltage at point D would closely approximate the true .average of the" positive voltage at point A."

From the foregoing-it will be-seen that capacito'r'5 2 operates, through resistor 56 and clamp diode 60, to

hold point C at approximately logic ground during the entire off time of the transistor switch 22. Accordingly, v

the signal at point C is truly the analogof the isignal applied to the input of the LC filters of the load outputs, such as the signal at point F, FIG. 1, whereby the de- 20 the'inv'ention.

1 described with reference toa preferred embodiment thereoflit will be understood by those skilled in the art .thatvarious changes in form and detail may be made therein without departing from the spirit and scope of What is claimed is:

l.'ln atransformer coupled transistorswitching regu- I later having a plurality of output circuits includinga't least one rectified load' circuit havingan inductor; 25 capacitoraveraging filterjand a feedba ck sense circuit forcontrol of switching transistor means'of said transistor switching regulator,-

the improvement co'mpris ing", f. i

said sense'circuit compris jn'g'a hair-wave, rectif er and a resistor-capacitor filtercomprising a-filter resistor having a first terminal connected to receiveenergy through said rectifier when saidlswitching' transis Liter,

with fs ai dloutputcircuit of said filters.

- and means operative. when said switching transistor datum potential "is switching transistor-'means. i'

5. Apparatusingaecordance with claim liwherein said regulator includesan input'cir'cjuitf controlled by said switchingtransistormeans 'and'f' sense circuit,

minal of said resistor.

I tormeans is conducive and aisecond terminal, said I f 1 resistor being-coupled by said terminals in series I with the output circuit of said 'resistor-capacitorfil- .a capacitor-coupled to saidsecond ternijnal of said resistor, said'capacitor'being connected in parallel means is-nonco nductive to applyadatum potential j inpcircuit with lsaid firstjtermihal o'fsaid resistor, saiddatu-m potential corresponding to the volt-time average of th'e' -tull wave signalappliedtofsaidrecti- .2..'.Appar atus.in reads-ma with claim 'whe' 'iii satay the. logic return potential level of cordancewithclaim lwherein said operative means comprisesapotential source applied a .'to said firsta terminal"of said 'resistor in the "absence of an input jofja predetermined polarity to said sense cir{ r m accordancewithclaihi wherein said inductive means coupling saidfinpu't circuit to said said sense circuit includihg'polarity resp'onsiyje means operative to couple signals'of said 'predetermined polarity from said inductive meansjtoflsaidfirst' ter f said storage means comprises a second capacitorfand' said unidirectional element being arranged to opera- ;said means to charge comprises unidirectional; -tively-couplesaidfirsttermin al of said filter resistor means connectedfin series with said second cap'aci-f; to they input of. said circuit during said first polarity @tor across said inductiyemeans and poled to'charge i of said inpu't,,said second terminal of said resistor 'ffl sai dfsecond capacitor during reyerseppolari ty;o'ut- 5 beingcoupled to said first terminal of said capaciputof'sai'd inductive means; r tor, and said capacitor being coupled by its first and impedanc connectionbetween saidpotential fand-second terminals across the output 'of said'fils ourceicomprises an impedance connectio'npbe qt er 4 a I tweenysaid'icapacitor andsaid first terminal of 'said f asecondunidirectional element 'and a second capacifilter resisted-f aud clamping means operativeg'to io w-;'-tor connected in series across said input, to charge IlimitIGVCISbXQUl'SiQI) 'Of Said first terminal of said j said-capacitor in the Second polarity during appliresistor substantiallyto sjaid datum potential 4 cation'of'potentialof said second polarity to said 7.'half-waveHpulseaveraging circuit adaptedjto j',." 'input-, Y I q espsond;ajfull-via've signal comprising, I ""irnped ance means havingia first terminal'and a sec- 'a"re'sis tor c apacitor filter comprising input polarity 151" "end terminal,' said first terminal of said impedance' control means, means-beingIcoupled'to said-. se cond capacitorrata {filter resistor means connected-by-afirst terminalto i circuitpo'int between said-second capacitor and vreceive energy through said coiit'rolmeanswhen an aid; second.iin idirec'tionalelement, andsaid see-I "input signalof a, first polarity is applied tosaid filend" terminal of saidzimpedaiice.meansbeing co'uter'; I I V I pl'edfto 'said first-terminalofjSaid-filter}resistor, f aid're'sistor means being coupled in 'se ries-withthe I nd cigmpim n co "e'cjtedsbetween-said second I output circuit of said fi' lter f capacitor m'eans coupled to said resistor, infp"r'allel with theoutputof said, filter, I I andfmeans.ope'rativefwhen an input signal of a second polarity =i sapplie dit o saidl filter, to'apply a datum-- v, potential in@circu'itwith said-first terinina'lfof said? I resistor means's'aid datum potential corresponding 1; to thesb' I slefyalue o f:th e' par'tial wa'\/.S 'gn'al passed,- l

- by said-input polarity-controlmean Iv 8. A pulsea eragirig cir Iit for us'ewithan input'hav" j" ingpotentialw'aveforrn portionsof first andsecondl j-'larities,during-first andyseendpo' ionsof Circuit'op ration, saidcircuit comprising- I saidthirds resistor-{being connecteldgac osss'aid'filter apac 'it'or Iandconstituting' 'with said afu-nidirectbnal condu tive elemera filter resistorhavingl fir'standsecond;term nals a filter capacitor having t ndsecond terminals filterresistora UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,859,590

DATED Jan. 7, 1975 INVENTOR) John Robert Cielo; William Albert Moorman;

Suryanarayana Krishnappa Rao It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, Line 7, "impedance connection between" should read ---the means to apply Signed and Scaled this Nineteenth Day of October 1976 [SEAL] RUTH C. MASON C. MARSHALL DANN Attestt'ttg Officer (umtrtissiuner of Patents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO.: 3,859,590

DATED Jan. 7, 1975 INVENTOR) John Robert Cielo; William Albert Moorman;

' Suryanarayana Krishnappa Rao It is certified that error appears in the above-identified patent and that said Letters Patent Q are hereby corrected as shown below:

Column 7, Line 7, "impedance connection between" should read --the means to apply-- Signed and Sealed this Nineteenth Day Of October 1976 [SEAL] AIIBSI.

RUTH C. MASON C. MARSHALL DANN Arresting Officer (mnmissinner nflarents and Trademarks 

1. In a transformer coupled transistor switching regulator having a plurality of output circuits including at least one rectified load circuit having an inductor-capacitor averaging filter and a feedback sense circuit for control of switching transistor means of said transistor switching regulator, the improvement comprising, said sense circuit comprising a half-wave rectifier and a resistor-capacitor filter comprising a filter resistor having a first terminal connected to receive energy through said rectifier when said switching transistor means is conducive and a second terminal, said resistor being coupled by said terminals in series with the output circuit of said resistor-capacitor filter, a capacitor coupled to said second terminal of said resistor, said capacitor being connected in parallel with said output circuit of said filter, and means operative when said switching transistor means is nonconductive to apply a datum potential in circuit with said first terminal of said resistor, said datum potential corresponding to the volt-time average of the full-wave signal applied to said rectifier.
 2. Apparatus in accordance with claim 1 wherein said datum potential is the logic return potential level of said regulator.
 3. Apparatus in accordance with claim 1 wherein said operative means comprises a potential source applied to said first terminal of said resistor in the absence of an input of a predetermined polarity to said sense circuit.
 4. Apparatus in accordance with claim 3 wherein said potential source comprises energy storage means and means to said storage means during the off time of said switching transistor means.
 5. Apparatus in accordance with claim 4 wherein said regulator includes an input circuit controlled by said switching transistor means, and inductive means coupling said input circuit to said sense circuit, said sense circuit including polarity responsive means operative to couple signals of said predetermined polarity from said inductive means to said first terminal of said resistor.
 6. Apparatus in accordance with claim 5 wherein, said storage means comprises a second capacitor and said means to charge comprises unidirectional means connected in series with said second capacitor across said inductive means and poled to charge said second capacitor during reverse polarity output of said inductive means, and impedance connection between said potential source comprises an impedance connection between said capacitor and said first terminal of said filter resistor, and clamping means operative to limit reverse excursion of said first terminal of said resistor substantially to said datum potential.
 7. A half-wave pulse averaging circuit adapted to respsond a full-wave signal comprising, a resistor-capacitor filter comprising input polarity control means, filter resistor means connected by a first terminal to receive energy through said control means when an input signal of a first polarity is applied to said filter, said resistor means being coupled in series with the output circuit of said filter, capacitor means coupled to said resistor, in parallel with the output of said filter, and means operative when an input signal of a second polarity is applied to said filter, to apply a datum potential in circuit with said first terminal of said resistor means, said datum potential corresponding to the base value of the partial wave signal passed by said input polarity control means.
 8. A pulse averaging circuit for use with an input having potential waveform portions of first and second polarities during first and second portions of circuit operation, said circuit comprising, a unidirectional conductive element, a filter resistor having first and second terminals, a filter capacitor having first and second terminals, said unidirectional element being arranged to operatively couple said first terminal of said filter resistor to the input of said circuit during said first polarity of said input, said second terminal of said resistor being coupled to said first terminal of said capacitor, and said capacitor being coupled by its first and second terminals across the output of said filter, a second unidirectional element and a second capacitor Connected in series across said input, to charge said capacitor in the second polarity during application of potential of said second polarity to said input, impedance means having a first terminal and a second terminal, said first terminal of said impedance means being coupled to said second capacitor at a circuit point between said second capacitor and said second unidirectional element, and said second terminal of said impedance means being coupled to said first terminal of said filter resistor, and clamp means connected between said second terminal of said filter capacitor and said first terminal of said resistor, said clamp means comprising a third unidirectional element poled to conduct when a potential of said second polarity is applied to said first terminal of said filter resistor by the conduction through said impedance means.
 9. Apparatus in accordance with claim 8, wherein said unidirectional elements are diodes, and said impedance means comprises a second resistor.
 10. Apparatus in accordance with claim 9, further including, a third resistor, said third resistor being connected across said filter capacitor and constituting with said filter resistor a voltage divider. 